Scheduling policies

The scheduler is the kernel component that decides which runnable thread will be executed by the CPU next. Each thread has an associated scheduling policy and a static scheduling priority, sched_priority; these are the settings that are modified by sched_setscheduler(). The scheduler makes it decisions based on knowledge of the scheduling policy and static priority of all threads on the system.

For threads scheduled under one of the normal scheduling policies (SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in scheduling decisions (it must be specified as 0).

Processes scheduled under one of the real-time policies (SCHED_FIFO, SCHED_RR) have a sched_priority value in the range 1 (low) to 99 (high). (As the numbers imply, real-time threads always have higher priority than normal threads.) Note well: POSIX.1-2001 requires an implementation to support only a minimum 32 distinct priority levels for the real-time policies, and some systems supply just this minimum. Portable programs should use sched_get_priority_min(2) and sched_get_priority_max(2) to find the range of priorities supported for a particular policy.

Conceptually, the scheduler maintains a list of runnable threads for each possible sched_priority value. In order to determine which thread runs next, the scheduler looks for the nonempty list with the highest static priority and selects the thread at the head of this list.

A thread's scheduling policy determines where it will be inserted into the list of threads with equal static priority and how it will move inside this list.

All scheduling is preemptive: if a thread with a higher static priority becomes ready to run, the currently running thread will be preempted and returned to the wait list for its static priority level. The scheduling policy determines the ordering only within the list of runnable threads with equal static priority.

SCHED_FIFO: First in-first out scheduling

SCHED_FIFO can be used only with static priorities higher than 0, which means that when a SCHED_FIFO threads becomes runnable, it will always immediately preempt any currently running SCHED_OTHER, SCHED_BATCH, or SCHED_IDLE thread. SCHED_FIFO is a simple scheduling algorithm without time slicing. For threads scheduled under the SCHED_FIFO policy, the following rules apply:

No other events will move a thread scheduled under the SCHED_FIFO policy in the wait list of runnable threads with equal static priority.

A SCHED_FIFO thread runs until either it is blocked by an I/O request, it is preempted by a higher priority thread, or it calls sched_yield(2).

SCHED_RR: Round-robin scheduling

SCHED_RR is a simple enhancement of SCHED_FIFO. Everything described above for SCHED_FIFO also applies to SCHED_RR, except that each thread is allowed to run only for a maximum time quantum. If a SCHED_RR thread has been running for a time period equal to or longer than the time quantum, it will be put at the end of the list for its priority. A SCHED_RR thread that has been preempted by a higher priority thread and subsequently resumes execution as a running thread will complete the unexpired portion of its round-robin time quantum. The length of the time quantum can be retrieved using sched_rr_get_interval(2).

SCHED_OTHER: Default Linux time-sharing scheduling

SCHED_OTHER can be used at only static priority 0. SCHED_OTHER is the standard Linux time-sharing scheduler that is intended for all threads that do not require the special real-time mechanisms. The thread to run is chosen from the static priority 0 list based on a dynamic priority that is determined only inside this list. The dynamic priority is based on the nice value (set by nice(2) or setpriority(2)) and increased for each time quantum the thread is ready to run, but denied to run by the scheduler. This ensures fair progress among all SCHED_OTHER threads.

SCHED_BATCH: Scheduling batch processes

(Since Linux 2.6.16.) SCHED_BATCH can be used only at static priority 0. This policy is similar to SCHED_OTHER in that it schedules the thread according to its dynamic priority (based on the nice value). The difference is that this policy will cause the scheduler to always assume that the thread is CPU-intensive. Consequently, the scheduler will apply a small scheduling penalty with respect to wakeup behaviour, so that this thread is mildly disfavored in scheduling decisions.

This policy is useful for workloads that are noninteractive, but do not want to lower their nice value, and for workloads that want a deterministic scheduling policy without interactivity causing extra preemptions (between the workload's tasks).

SCHED_IDLE: Scheduling very low priority jobs

(Since Linux 2.6.23.) SCHED_IDLE can be used only at static priority 0; the process nice value has no influence for this policy.

This policy is intended for running jobs at extremely low priority (lower even than a +19 nice value with the SCHED_OTHER or SCHED_BATCH policies).

Resetting scheduling policy for child processes

Since Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed in policy when calling sched_setscheduler(). As a result of including this flag, children created by fork(2) do not inherit privileged scheduling policies. This feature is intended for media-playback applications, and can be used to prevent applications evading the RLIMIT_RTTIME resource limit (see getrlimit(2)) by creating multiple child processes.

More precisely, if the SCHED_RESET_ON_FORK flag is specified, the following rules apply for subsequently created children:

After the SCHED_RESET_ON_FORK flag has been enabled, it can be reset only if the thread has the CAP_SYS_NICE capability. This flag is disabled in child processes created by fork(2).

The SCHED_RESET_ON_FORK flag is visible in the policy value returned by sched_getscheduler()

Privileges and resource limits

In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) threads can set a nonzero static priority (i.e., set a real-time scheduling policy). The only change that an unprivileged thread can make is to set the SCHED_OTHER policy, and this can be done only if the effective user ID of the caller of sched_setscheduler() matches the real or effective user ID of the target thread (i.e., the thread specified by pid) whose policy is being changed.

Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling on an unprivileged thread's static priority for the SCHED_RR and SCHED_FIFO policies. The rules for changing scheduling policy and priority are as follows:

Privileged (CAP_SYS_NICE) threads ignore the RLIMIT_RTPRIO limit; as with older kernels, they can make arbitrary changes to scheduling policy and priority. See getrlimit(2) for further information on RLIMIT_RTPRIO.

Response time

A blocked high priority thread waiting for the I/O has a certain response time before it is scheduled again. The device driver writer can greatly reduce this response time by using a "slow interrupt" interrupt handler.

Miscellaneous

Child processes inherit the scheduling policy and parameters across a fork(2). The scheduling policy and parameters are preserved across execve(2).

Memory locking is usually needed for real-time processes to avoid paging delays; this can be done with mlock(2) or mlockall(2).

Since a nonblocking infinite loop in a thread scheduled under SCHED_FIFO or SCHED_RR will block all threads with lower priority forever, a software developer should always keep available on the console a shell scheduled under a higher static priority than the tested application. This will allow an emergency kill of tested real-time applications that do not block or terminate as expected. See also the description of the RLIMIT_RTTIME resource limit in getrlimit(2).

POSIX systems on which sched_setscheduler() and sched_getscheduler() are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>

Real-time features in the mainline Linux kernel

From kernel version 2.6.18 onward, however, Linux is gradually becoming equipped with real-time capabilities, most of which are derived from the former realtime-preempt patches developed by Ingo Molnar, Thomas Gleixner, Steven Rostedt, and others. Until the patches have been completely merged into the mainline kernel (this is expected to be around kernel version 2.6.30), they must be installed to achieve the best real-time performance. These patches are named:

and can be downloaded from http://www.kernel.org/pub/linux/kernel/projects/rt/

Without the patches and prior to their full inclusion into the mainline kernel, the kernel configuration offers only the three preemption classes CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, and CONFIG_PREEMPT_DESKTOP which respectively provide no, some, and considerable reduction of the worst-case scheduling latency.

With the patches applied or after their full inclusion into the mainline kernel, the additional configuration item CONFIG_PREEMPT_RT becomes available. If this is selected, Linux is transformed into a regular real-time operating system. The FIFO and RR scheduling policies that can be selected using sched_setscheduler() are then used to run a thread with true real-time priority and a minimum worst-case scheduling latency.